A newly discovered dwarf galaxy in the Local Group has been found to have formed in a region of space far from our own and is falling into our system for the first time in its history, according to new data obtained at the W. M. Keck Observatory. An international team of astronomers report that the dwarf galaxy, Andromeda XII, marks the best piece of evidence for small galaxies which are just now arriving in our Local Group. The finding provides an important test for simulations of galaxy formation.
Dwarf galaxies and streams of stellar material mark the visible remnants of galactic merging events from which large galaxies are made. Cosmology models predict small galaxies form along a web of filamentary structures in the universe, and then gradually fall into dense groups and cluster environments. Small galaxies should still be falling into the Local Group, yet none have been found--until now.
“Other Local Group dwarf galaxies are thought to have extreme orbits, including Leo I, Andromeda XIV and Andromeda XI, but Andromeda XII really stands out as a contender for a new entrant into the Local Group,” said the lead author of the study, Dr. Scott C. Chapman of the University of Cambridge, Institute of Astronomy. “The others have likely already been seriously harassed by Andromeda and the Milky Way.”
Nicknamed the ‘Olympian Galaxy’ after the Twelve Olympians in the Greek Pantheon, Andromeda XII was first discovered in October 2006 during a wide-field survey taken with the Canada-France Hawaii Telescope’s “MegaCam” instrument. It is the faintest dwarf galaxy ever discovered near to Andromeda (M31), and may have the lowest mass ever measured. Dwarf galaxies are the smallest stellar systems showing evidence for a substantial amount of dark matter.
Dr. Chapman’s observations confirmed Andromeda XII is distinct from all other satellite galaxies in the Local Group. It is a fast-moving galaxy on a highly eccentric orbit, located at a great distance from the center of M31, about 115 kiloparsecs (375,000 light years). Importantly, Andromeda XII lies significantly behind M31 as viewed from the Milky Way, almost certainly falling in for the first time. Because Andromeda XII has lived its life in a very different environment than the Local Group, it gives astronomers a pristine object for studying star formation histories, dark matter distribution, and other parameters that would be influenced by the Local Group gravity that has affected all the in other dwarf galaxies.
“Andromeda XII may be the first galaxy of the local group ever observed that has not yet been disrupted by the strong gravity of the Local Group,” said Dr. Jorge Penarrubia of the University of Victoria, a co-author of the study.
The DEIMOS spectrograph at Keck II, one of two 10-meter telescopes the W. M. Keck Observatory operates on the summit of Mauna Kea, was key in making the discovery. It was used to observe 49 stars in the region of Andromeda XII, and confirmed that eight were members of the new dwarf galaxy. Follow-up observations were also conducted at the Green Bank Telescope in West Virginia to measure the amount of interstellar gas in the galaxy, and the Subaru telescope in Hawaii helped determine a more precise distance.
“Without the spectra we obtained with DEIMOS, it would have been impossible to make any useful claims about the orbit of Andromeda XII, its evolution, its speed or its dark matter content,” added Dr. Chapman.
Andromeda XII is falling very quickly through the Local Group from behind Andromeda, the only one of Andromeda’s satellites which exceeds the apparent escape velocity for Andromeda. It is possible that Andromeda XII may be just a short-term visitor. It is such a low-mass galaxy that it may not slow down much as it passes through the Local Group.
“It is a pleasure to see the speed of this new, fascinating member of the Local Group clocked using Keck II and DEIMOS,” added W. M. Keck Observatory Director Taft Armandroff. “The powerful combination of Keck and DEIMOS has added many contributions to our understanding of Local Group galaxies.”
The age of the Universe is not old enough for Andromeda XII to have started in the dense Local Group and be on its second trip through our system. Andromeda XII probably formed in a dense filament structure, toward the general direction of the M81 group. However, the distance is about three times too large for it to have actually come from the M81 group. A likely scenario is Andromeda XII formed in a filamentary region of space that connects the Local Group to the M81 group.
“The high speed of Andromeda XII really surprised me; I wasn’t expecting to see any of our newly discovered dwarfs moving so fast. We will likely have to revise our mass estimates of Andromeda upward as a result.” added Rodrigo Ibata.
A paper reporting the discovery, “Strangers in the Night: The discovery of a Dwarf Spheroidal Galaxy on its First Local Group Infall,” will appear in an upcoming issue of the Astrophysical Journal. Funding was provided by a fellowship from the Canadian Space Agency and the Natural Sciences and Engineering Research Council of Canada. Additional support was provided by Adrian Jenkins who provided use of important computer simulations.
The study was co-authored by Jorge Penarrubia, Alan McConnachie, Aaron Ludlow of the University of Victoria; Rodrigo Ibata, Observatoire de Strasbourg; Nicolas F. Martin, Max-Planck Institut fur Astronomie; Andrew Blain and Bruno LeTarte, California Institute of Technology; Michael Irwin, University of Cambridge Institute of Astronomy; Geraint Lewis, University of Sydney Institute of Astronomy; Fred Lo and Karen O’Neil, NRAO Green Bank Telescope.
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